This invention relates to semiconductor devices. More particularly, it is concerned with methods of manufacturing junction field effect transistors of the static induction type.
The static induction transistor is a field effect semiconductor device capable of operation at relatively high frequency and power. The transistors are characterized by a short, high resistivity semiconductor channel region which may be controllably depleted of carriers. The current-voltage characteristics of the static induction transistor are generally similar to those of a vacuum tube triode.
The static induction transistor generally uses vertical geometry with source and drain electrodes placed on opposite sides of a thin, high resistivity layer of silicon of one conductivity type. Gate regions of the opposite conductivity type are positioned in the high resistivity layer on opposite sides of the source. During operation a reverse bias is applied between the gate regions and the remainder of the high resistivity layer causing a depletion region to extend into the channel region below the source. As the magnitude of the reverse bias is varied, the source-drain current and voltage derived from an attached energy source will also vary.
In surface gate static induction transistors, silicon oxide at the surface of the high resistivity layer isolates the alternating source and gate regions from each other. This silicon oxide should be of sufficient thickness to prevent excessive capacitance between the underlying silicon and overlying electrical connections. The silicon oxide is thermally grown by exposing silicon to oxygen while protecting the surface areas of the future source and gate regions with silicon nitride. During the thermal oxidation process, however, oxygen diffuses partially under the edges of the silicon nitride increasing the horizontal dimensions of the isolating silicon oxide and altering the horizontal dimensions of the source and gate surface areas from those which were defined by the silicon nitride. Thus, it is difficult to control, and therefore to reduce, the gate-to-gate distance, or pitch, which is critical in determining the operating characteristics of the device.